Uptake, cytotoxicity, neoplastic transformation and oncogene expression, induced by samples of crystalline silica (<5 microns in diameter) were studied in the BALB/3T3/A31-1-1 mouse embryo cell line. Particulates, further characterized for surface properties and size, included several samples of quartz, cristobalite and tridymite, and non-fibrogenic dusts for comparison. Uptake and cytotoxicity were studied by serially timed phase-contrast photography, and by scanning and transmission electronmicroscopy (E.M.). Cell monolayers, previously exposed to silica or unexposed, received a dose of silica at 2 weeks: cell lysis developed slowly and peaked at 3-4 days, followed by regrowth of confluent monolayers by 10 days; the response was the same in previously untreated and silica-pretreated cells, indicating that silica toxicity did not result in selection of resistant cells. E.M. studies showed uptake of silica particles, sometimes engulfed by pseudopodia, and internalized mostly in membrane-bound vesicles; destruction of cytoplasmic organelles was found around some particles. Some particles were found in contact with the nuclear membrane; very few were inside nuclei. Repeated transformation assays showed a dose dependent induction of transformed foci up to a dose of 25-50 microg/cm2, followed by a plateau indicative of a rate-limiting factor possibly related to uptake. A quartz sample, F600, showed minimal cytotoxicity, but the same level of transforming activity as other more toxic quartz samples. All the transformed foci induced by tested quartz samples were rapidly tumorigenic in nude mice. Karyotypic analysis of 9 silica-transformed cell lines (7-8 metaphases each) showed that all had acquired one or more marker chromosomes not seen in untreated cells or in spontaneous transformants. Total RNA isolated from silica- induced foci, analyzed by slot blot hybridization with 10 probes for different oncogenes and for p53, showed increased expression of myc, H- ras, K-ras, abl and p53 in cells transformed by MQZ and HFMQZ, but not by F600. Expression of these genes was further increased in the corresponding tumors in nude mice.